17.2 Seafloor Spreading

Objectives• Summarize the evidence that led to the

discovery of seafloor spreading.• Explain the significance of magnetic patterns

on the seafloor.• Explain the process of seafloor spreading.

– magnetometer– paleomagnetism– magnetic reversal

– isochron– seafloor spreading

Vocabulary

Seafloor Spreading

• Many thought the ocean floor was flat until the mid-1900s.

• Many thought that oceanic crust was unchanging and was much older than continental crust.

• Technology in the 40’s 50’s,showed those ideas to be wrong.

SONAR- Sound Navigation and Ranging

• Sound waves could find the depth of the ocean floor.

• Sound waves travel through water at: – R = 1482 m/s– T= was measured– D= was found

• R×T = D

Magnetometer• A magnetometer is a

device that can detect small changes in magnetic fields.

Ocean Floor Topography• Maps made from sonar and magnetometers

showed underwater mountain chains called ocean ridges.

• Deep-sea trenches were found.

• Geologists could not explain why there were trenches and mountains on the oceans floor.

Trenches

Mid-Atlantic Ridge

• mountains

Ocean Rocks and Sediments

Analysis of deep-sea rocks and sediments found

1. Ages of the seafloor rocks differ. The farther from a ridge the older the rock.

– The oldest part of the seafloor is geologically young at about 180 million years old.

2. Ocean-floor sediment gets thicker farther from a ridge

Ocean Rocks and Sediments

• Rocks containing iron-bearing minerals provide a record of Earth’s magnetic field.

• Paleomagnetism is the study of Earth’s magnetic record.

• Basalt, because it is rich in iron-bearing minerals, provides an accurate record of ancient magnetism.

• A magnetic field that is the same as the present has normal polarity.

• A magnetic field that is opposite to the present has reversed polarity.

MagnetismThe Geomagnetic Time Scale

– Studies reveal a pattern of magnetic reversals over geologic time.

– A magnetic reversal is a change in Earth’s magnetic field.

• In places where the magnetic readings of the ocean floor matched Earth’s present field, a stronger-than-normal reading (+) was recorded.

• In places where the magnetic data were reversed in relation to Earth’s present magnetic field, a lower-than-normal reading (–) was recorded.

The Geomagnetic Time Scale

– Towing magnetometers behind ships revealed an interesting magnetic pattern.

The Geomagnetic Time Scale

Magnetic Symmetry

– The positive and negative areas of the seafloor form a series of stripes that were parallel to ocean ridges.

– The magnetic pattern on one side of the ridge is a mirror image of the pattern on the other side of the ridge.

Magnetic Symmetry

– The magnetic data matched the pattern that had been found in basalt flows on land.

– This allowed scientist to determine the age of the ocean floor. And make isochron maps.

– An isochron is a line on a map that connects

points that have the same age.

Seafloor Spreading• An American scientist named Harry Hess proposed

the theory of seafloor spreading.

– Magma is forced toward the crust along an ocean ridge and fills the gap that is created.

• Seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.

Seafloor Spreading

– Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge.

– When the magma hardens, a small amount of new ocean floor is added to Earth’s surface.

The Missing Link

– Seafloor spreading was the missing link to complete his model of continental drift.

– Continents are not pushing through ocean crust, as Wegener proposed; they ride with ocean crust as it slowly moves away from ocean ridges.

Section Assessment

1. Match the following terms with their definitions.___ magnetometer

___ paleomagnetism

___ isochron

___ seafloor spreading

A. a device that can detect small changes in magnetic fields

B. a line on a map that connects points that have the same age

C. the study of Earth’s magnetic record

D. a theory that states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches

A

C

B

D

Section Assessment2. How does the distribution of ocean-floor sediments

support the theory of seafloor spreading?

The thickness of ocean-floor sediments increases with distance from an ocean ridge which indicates that the seafloor is older with distance.